Helical growth of semiconducting columnar dye assemblies based on chiral perylene bisimides

Article abstract of DOI:10.1021/ol0700963

(Figure Presented) A perylene bisimide derivative bearing two phenyl substituents with chiral solubilizing alkyl chains at the imide N atoms has been synthesized, and its self-assembly properties in solution and condensed phase have been investigated. Tem

Full text of DOI:10.1021/ol0700963

ORGANIC
LETTERS
2007
Vol. 9, No. 6
1085-1088
Helical Growth of Semiconducting
Columnar Dye Assemblies Based on
Chiral Perylene Bisimides
Volker Dehm,† Zhijian Chen,† Ute Baumeister,‡ Paulette Prins,§
Laurens D. A. Siebbeles,§ and Frank Wurthner*,†
1
Institut fu¨r Organische Chemie and Ro¨ntgen Research Center for Complex Material
Systems, UniVersita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany, Institut fu¨r
Physikalische Chemie, Martin-Luther-UniVersita¨t Halle-Wittenberg, Mu¨hlpforte 1,
06108 Halle, Germany, and Opto-electronic Materials Section, DelftChemTech,
Delft UniVersity of Technology, NL-2629 JB Delft, The Netherlands
wuerthner@chemie.uni-wuerzburg.de
Received January 12, 2007
ABSTRACT
A perylene bisimide derivative bearing two phenyl substituents with chiral solubilizing alkyl chains at the imide N atoms has been synthesized,
and its self-assembly properties in solution and condensed phase have been investigated. Temperature-dependent CD spectra revealed the
coexistence of two different kinds of chiral aggregates, differing in size and handedness. The chiral side chains effect a higher order within
the self-assemblies, resulting in an increased charge-carrier mobility in the columnar liquid crystalline mesophase.
Helical self-organization by noncovalent interactions is a
widely observed feature of natural biomacromolecules that
directs the formation of highly ordered structures, e.g., the
spontaneous self-assembly of DNA into a double helix and
of proteins and polysaccharides into R-helices. Inspired by
the unique features of fascinating biological superstructures,
numerous artificial assemblies have been developed in the
past decades by utilizing noncovalent forces such as hydro-
gen bonding, metal ion coordination, and π-π interaction.1
Recently, several groups have initiated programs to engineer
functional materials for electronic devices by proper control
of the packing of π-conjugated molecules by such nonco-
valent interactions.2-5 In particular, various examples of
discotic molecules such as triphenylenes,3 hexabenzocoro-
nenes,4 and phthalocyanines5 have been reported to form π
stacks with helical superstructures that exhibit increased
charge-carrier mobilities as a result of the higher molecular
order imparted by the helical stacking arrangement.
Although all of these above-mentioned molecules possess
electron-rich aromatic cores that give rise to columnar π
stacks with p-type semiconducting properties, we have
(2) (a) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya,
I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp,
A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature 2002, 417, 384. (b)
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For a recent review, see: Schenning, A. P. H. J.; Meijer, E. W. Chem.
Commun. 2005, 3245.
(3) Adam, D.; Schumacher, P.; Simmerer, J.; Ha¨usling, L.; Siemensm-
eyer, K.; Etzbach, K. H.; Ringsdorf, H.; Harrer, D. Nature 1994, 371, 141.
(4) Wu, J.; Watson, M. D.; Zhang, L.; Wang, Z.; Mu¨llen, K. J. Am.
Chem. Soc. 2004, 126, 177.
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785.
† Universita¨t Wu¨rzburg.
‡ Martin-Luther-Universita¨t Halle-Wittenberg.
§ Delft University of Technology.
(1) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk,
N. A. J. M. Chem. ReV. 2001, 101, 4039.
10.1021/ol0700963 CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/15/2007

Much to our surprise, the self-assembly studies of this chiral
building block in apolar aliphatic solutions revealed some
striking aspects with regard to the development of chiral
order upon columnar growth, and the condensed liquid
crystalline (LC) phase of 3 showed an improved charge-
carrier mobility.
Scheme 1. Synthesis of the Chiral PBI 3
The PBI derivative 3 with six appended chiral side chains
was synthesized in five steps starting with the known
aldehyde 49 according to Scheme 1. The details of the
synthesis are given in Supporting Information.
The self-assembly behavior of PBI dye 3 in solution was
studied by UV/vis absorption spectroscopy in methylcyclo-
hexane (MCH). As shown in Figure 1, at low concentrations
recently discovered that also the considerably elongated
electron-poor π-electron system perylene bisimides (PBI),
e.g., the derivatives 1 and 2, preferentially stack in a
columnar fashion due to rotational displacements of neigh-
boring molecules.6,7 However, owing to the achiral nature
of these PBI building blocks, no chiral supramolecular
ordering could be observed on a macroscopic level; rather,
disordered hexagonal columnar mesophases were found.
In the absence of chiral units in these PBI molecules, both
possible helical conformations (i.e., M and P) necessarily
have equal intermolecular interaction energies; therefore,
racemic mixtures of helical assemblies can be expected.
However, by means of chiral side chains, the M/P chemical
equilibrium can be biased, which affords a preferential
helicity on the higher supramolecular level of organization.8
Thus, we envisioned that PBI derivatives containing chiral
side chains should afford nonracemic helical supramolecular
structures.
Figure 1. Concentration-dependent UV/vis absorption spectra of
PBI 3 (10-6 to 2 × 10-3 M) in MCH at 25 °C. Arrows indicate
changes upon increasing concentration.
an absorption band with a well-resolved vibrational fine
structure between 400 and 530 nm was observed that can
be attributed to the S0 f S1 transition of the perylene
bisimide monomer.
The absorption maximum appeared at 517 nm, along with
two higher vibronic transitions located at 482 and 451 nm.
Upon increasing the concentration, the absorption coefficient
dramatically decreased and a blue shift of the absorption
maximum to 491 nm was observed. In addition, a second
red-shifted absorption band of lower intensity at 536 nm
evolved. Owing to the presence of well-defined isosbestic
points at 469, 502, and 529 nm, we may be tempted to
conclude that only two different kinds of absorbing species
are in equilibrium. The observed spectral changes at higher
concentrations can be attributed to excitonic coupling
between closely π-π-stacked perylene bisimides with
rotational displacements of about 55° between the neighbor-
ing dyes.10 Temperature-dependent UV/vis spectra (Figure
S1 in Supporting Information) reveal similar spectral features
as observed in the concentration-dependent measurements.
This suggests that the spectral changes originate from the
same process, i.e., self-assembly of monomeric dyes into
larger aggregates composed of π-π-stacked species.
By means of concentration- and temperature-dependent
UV/vis spectroscopy for 3 in MCH (from 10 to 70 °C in
intervals of 10 °C), we were able to determine the enthalpy
Here, we present a new chiral PBI derivative 3, which
indeed affords helical superstructures upon self-assembly.
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E. W. Angew. Chem., Int. Ed. 1997, 36, 2648. (b) Brunsveld, L.; Zhang,
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A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2004, 126, 10021. (d) Nakade,
H.; Jordan, B. J.; Xu, H.; Han, G.; Srivastava, S.; Arvizo, R. R.; Cooke,
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F. Chem. Phys. 2006, 328, 354.
1086
Org. Lett., Vol. 9, No. 6, 2007

and entropy values of ∆H° ) -58.9 ( 1.0 kJ mol-1 and
∆S° ) -107.9 ( 0.1 J mol-1 K-1 for this self-assembly
process from the linear van’t Hoff plot (Figure S2 in
Supporting Information). In our recent work,7 we have
reported in detail on the calculation of average aggregation
numbers N from UV/vis data of similar one-dimensional self-
assemblies of PBI derivative 2 based on the isodesmic model
(also referred to as the equal K model)11 and confirmed these
results by vapor pressure osmometry and DOSY NMR
studies. By applying this method to the concentration- and
temperature-dependent (from -10 to 90 °C) UV/vis spectral
data of PBI dye 3, we could estimate average aggregation
numbers (N) for a 2 × 10-3 M solution of 3 in MCH at
different temperatures. Thus, at -10, 40, and 90 °C, the N
values of 55, 6, and 2, respectively, were obtained. A
complete set of data are given in Table S2 in the Supporting
Information.
tion processes can be gained by CD spectroscopy.8,12 Thus,
the impact of the molecular chiral side chains of 3 on the
supramolecular order of its π-π-stacked aggregates was
examined by temperature-dependent CD spectroscopy (Fig-
ure 2a,b). No CD signal could be observed for the PBI 3
monomer in dichloromethane (DCM) solution between 350
and 650 nm, whereas for the aggregates of 3 in concentrated
MCH solution, a strong induced bisignate Cotton effect was
observed at -10 °C with a positive maximum at 548 nm
and a negative maximum at 498 nm (Figure 2a, green line).
Upon heating from -10 (green line) to 40 °C (red line),
the Cotton effect at 548 nm completely vanished, whereas
the one at 498 nm was diminished. On the other hand, a
positive CD band emerged at 462 nm and a negative shoulder
appeared at 512 nm. Further heating of the sample to 90 °C
(Figure 2b, blue line) resulted in an overall decrease of the
CD signal, but even at this high temperature, dimeric species
prevail as the bisignate CD signal with a negative maximum
at 500 nm and a positive maximum at 460 nm as well as the
UV/vis spectra (Figure 2c) reveal. The aggregation process
of 3 is fully reversible, as upon cooling the sample from 90
to -10 °C the initial CD spectrum is recovered.
On the basis of the N values given in Table S2 in the
Supporting Information, one can relate the CD spectrum at
90 °C to PBI dimers, whereas the CD spectrum at -10 °C
originates from extended species with average aggregation
numbers greater than 50. At intermediate temperatures,
complex CD spectra arise that lack defined isodichroic points.
This observation does not comply with the earlier conclusion
of a two-state equilibrium that was drawn based on well-
defined isosbestic points observed in UV/vis spectra. Ac-
cording to CD spectroscopy, an equilibrium among at least
three different species has to be considered to account for
the observed temperature-dependent features in the CD
spectra, as proposed in Figure 3.
Figure 2. Temperature-dependent CD and UV/vis spectra of 3 (2
× 10-3 M) in MCH. (a) CD spectra from -10 to 40 °C (green
line, -10 °C; black line, 10 °C; red line, 40 °C), in intervals of
10 °C. (b) CD spectra from 40 to 90 °C (red line, 40 °C; blue line,
90 °C), in intervals of 10 °C. Arrows indicate changes upon
increasing temperature. (c) UV/vis spectra at 10, 40, and 90 °C
(black, red, and blue line, respectively).
As can be seen in Figure 2c, the remarkable increase in
aggregate size, i.e., higher N values, upon cooling the sample
from 40 to 10 °C (N value increased from 6 to 34), does not
affect the shape of the UV/vis absorption spectrum signifi-
cantly. Thus, aggregates of different sizes, i.e., dimers,
trimers, etc., cannot be discriminated by this method. Only
at higher temperature, where the molar fraction of the
monomer increases, is a notable change in the shape of
absorption bands observed (blue line in Figure 2c).
Figure 3. Proposed equilibrium of monomer and aggregated
species of PBI 3 in solution. At low concentration and/or high
temperature, dimers are formed that are preferentially of M-type
helicity. Upon further growth into extended polymeric stacks,
P-configured species are preferred.
The low-temperature CD spectra indicate the preferential
formation of right-handed (P-configured) extended ag-
Recent studies on self-assembly of chiral π-conjugated
molecules have revealed that a deeper insight into aggrega-
(12) (a) Jonkheijm, P.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer,
E. W. Science 2006, 313, 80. (b) Ajayaghosh, A.; Varghese, R.; Goerge,
S. J.; Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141. (c) Lohr,
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(11) Martin, R. B. Chem. ReV. 1996, 96, 3043.
Org. Lett., Vol. 9, No. 6, 2007
1087

gregates according to the exciton chirality method,13 which
in the course of the fully reversible aggregation process
transform upon increasing temperature into preferentially left-
handed (M-configured) dimers. It is noteworthy that the
magnitude of the observed CD intensities remains moderate
in the investigated temperature range. Accordingly, despite
the biased chemical equilibrium toward dimers with M-
helicity and extended stacks with P-helicity, also their
diastereomeric counterparts coexist in solution. However,
upon decrease of temperature of PBI 3 solutions (Figure
2a,b), as well as for its thin films (Figure S3 in Supporting
Information), the intensity of the CD signal increases,
pointing to an increase of supramolecular order in the
columnar π stacks of the bulk material that is further
substantiated by dissymmetry factor g (∆ꢀ/ꢀ) values (Table
S3 in Supporting Information).
temperature, we obtained an isotropic TRMC lower mobility
limit of 0.010 cm2 V-1 s-1, which appeared to be constant
over a broad temperature range from room temperature to
at least 100 °C. Because this kind of material only transports
charge carriers along the columnar structures, a one-
dimensional TRMC lower mobility limit of 0.030 cm2 V-1
s-1 can be deduced and a one-dimensional TRMC mobility
of 0.087 cm2 V-1 s-1 could be estimated (for details see
Supporting Information). For comparison, we have also
determined the one-dimensional TRMC lower mobility limit
for the achiral PBI 2 in the Colhd phase (at 110 °C), which
is amounted to only 0.0078 cm2 V-1 s-1. Hence, the TRMC
mobility value for the LC state of 3 is about 4 times higher
than that for the Colhd mesophase of PBI 2. This can be
explained in terms of higher intracolumnar order in the LC
phase of 3, which is an outcome of the helical arrangement
imparted by the chiral side chains.
The thermotropic behavior of 3 was studied by differential
scanning calorimetry (DSC) and X-ray diffraction. The DSC
measurements (Figure S4 in Supporting Information) re-
vealed only one reversible phase transition from a LC phase
into the isotropic liquid phase at 349 °C with an enthalpy
change of ∆H ) 28 kJ mol-1 (19 J g-1). The ∆H value of
3 is significantly larger than those observed for PBI deriva-
tives 1 and 2 (9 and 19 kJ mol-1, respectively),6 implying a
higher-ordered LC phase of 3. This is also confirmed by a
higher entropy change ∆S ) 45 J mol-1 K-1 for 3 for the
isotropization process compared to 1 and 2 (14 and 33 J
mol-1 K-1, respectively). The higher order of the LC phase
for 3 was further substantiated by the X-ray diffraction
pattern (Figure S6, Table S4 in Supporting Information) of
a powder-like sample that can be interpreted in terms of a
columnar hexagonal ordered (Colho) mesophase with a cell
parameter of 3 nm and a defined intracolumnar distance of
3.5 Å between the chromophore planes. In contrast, disor-
dered columnar hexagonal mesophases were observed for 1
and 2. The above-mentioned results of DSC and X-ray
diffraction analyses of 3 confirm the presence of a highly
ordered mesophase, which exists over a very broad temper-
ature range from 0 °C to its clearing point at 349 °C.
The semiconducting properties of the π stacks of PBI 3
in this mesophase were investigated by using the pulse-
radiolysis time-resolved microwave conductivity (PR-
TRMC) technique.14,15 For the Colho LC phase of 3 at room
In conclusion, PBI dye 3 with appended chiral side chains
has provided valuable insight into the stepwise growth of
one-dimensional molecular aggregates in solution, and the
impact of helical order on charge-carrier mobilities for a
highly promising class of organic n-type semiconductors has
been shown. Taken together, from the present observations
and some other recent results from our and other laborato-
ries,12 we have learned that one should be cautious of drawing
premature and far-reaching conclusions with regard to the
uniformity of supramolecular species based on concentration-
and temperature-dependent UV/vis spectra with well-defined
isosbestic points.
Acknowledgment. Financial support by the Deutsche
Forschungsgemeinschaft within the research training school
GK 1221 is gratefully acknowledged.
Supporting Information Available: Synthesis and char-
acterization of PBI 3 and additional spectroscopic and
analytical data. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL0700963
(15) PR-TRMC charge-carrier mobilities have been reported for crystal-
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C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.; Kimkes, P.; Koehorst,
R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de Craats, A.
M.; Warman, J. M.; Zuilhof, H.; Sudholter, E. J. R. J. Am. Chem. Soc.
2000, 122, 11057. (b) Chen, Z.; Debije, M. G.; Debaerdemaeker, T.;
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J.; Jones, S. C.; Barlow, S.; Yoo, S.; Domercq, B.; Prins, P.; Siebbeles, L.
D. A.; Kippelen, B.; Marder, S. R. AdV. Mater. 2005, 17, 2580. (d) Debije,
M. G.; Chen, Z.; Piris, J.; Neder, R. B.; Watson, M. M.; Mu¨llen, K.;
Wu¨rthner, F. J. Mater. Chem. 2005, 15, 1270.
(13) Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism:
Principles and Applications; Wiley-VCH: New York, 2000.
(14) Warman, J. M.; de Haas, M. P.; Dicker, G.; Grozema, F. C.; Piris,
J.; Debije, M. G. Chem. Mater. 2004, 16, 4600.
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Org. Lett., Vol. 9, No. 6, 2007